Spatial inhomogeneities of the backscattering coefficient due to different aerosol fractions in the atmosphere were studied using an instrumented aircraft at altitudes up to 5 km. The resulting experimental data on the optical and microphysical aerosol properties were interpreted statistically to yield information on the average and fluctuation characteristics of the atmospheric backscattering coefficient. It was found that the average characteristics were determined by the submicron aerosol fraction number density, the fluctuations being largely caused by aerosol species with radii of >2 m.